Discussion
Our study demonstrates that PMN-MDSCs were proinflammatory and pathogenic in CIA and elucidates the supportive role of PMN-MDSCs to B cells through BAFF/ BTK/NF-๐›‹B signaling pathway, which suggested a potential pathological mechanism of CIA.
In our study, PMN-MDSCs expanded significantly in the CIA model and correlated positively with the arthritis score, which is consistent with previous studies(9, 10). However, some of our findings are contrary to those of the previous studies. Earlier studies found that PMN-MDSC adoptive transfer reduced the severity of CIA(8, 11), and administration of anti-Gr-1 Abs (MDSC depletion) abrogated the spontaneous improvement of CIA(7), while Guo et al.(9) found that MDSCs depletion could reduce disease severity, and we also found that anti-Ly6G Ab administration (PMN-MDSC depletion) accelerated the recovery of CIA. The different results of these studies could be attributed to the heterogeneity of MDSC.
B-cells are involved in the production of antibodies and cytokines. Andrey et al.(13) found that B-cell-derived TNF mediates the severity of CIA by controlling pathogenic autoantibody production. The success of anti-B cell therapy in RA has confirmed the pathological role of B cells(25). Therefore, it would be interesting to verify whether PMN-MDSCs can modulate B cell activity in autoimmune arthritis. Crook et al.(26) found that M-MDSCs from spleens of CIA mice, but not Ly6G+cells from the BM, inhibited B cell proliferation and antibody production, while Jang et al.(27) found that spleens derived from CD11b+Gr-1+ cells directly promoted the survival of plasma cells in a lupus-prone mouse model, and found that CD11b+Ly6G+ PMN-MDSCs from spleens of CIA could support B cells in vivo and in vitro . To our knowledge, it is the first demonstration that PMN-MDSCs from CIA mice not only support the B cellsโ€™ survival, but also promote TNF-๐›‚ secretion via BAFF, which was probably a potential pathogenesis of RA. Whatโ€™s more, TNF-๐›‚ was reported to activate BAFF release in RA(28), so the positive feedback between TNF-๐›‚ and BAFF was probably a significant promotor during the onset and peak of RA.
EAE is a disease model of MS with pathogenesis similar to that of CIA. Benjamin et al.(29) found that PMN-MDSCs from EAE mice during recovery inhibited B-cell proliferation, whereas PMN-MDSCs at disease onset had no such inhibitory effects. In our study, PMN-MDSCs were from CIA mice at peak; therefore, PMN-MDSCs from different stages of disease probably played distinct roles in the pathogenesis. Thus, the evolution of CD11b+Ly6G+ myeloid cells during disease development is worth exploring. PMN-MDSCs and neutrophils have similar morphological and phenotypic characteristics, but distinct functions. Recent research has reported that mouse PMN-MDSCs express more CD115 and CD244 than neutrophils(30); however, further studies are needed to confirm these markers. MDSC are known to be immunosuppressive; however, accumulating evidence suggests that MDSC are pathogenic in autoimmune diseases and their suppressive function is diminished(9, 31-33), therefore, it is more difficult to distinguish between neutrophils and PMN-MDSCs. Therefore, more studies are needed to identify a set of proper markers to distinguish PMN-MDSCs from neutrophils. In our study, we found a great expansion of PMN-MDSCs, but the potential mechanism is still unknown. PMN-MDSCs were considered to be immunosuppressive, especially in tumors, but we found that PMN-MDSCs could support B cells, suggesting that the microenvironment affects the function of PMN-MDSCs. Therefore, what causes PMN-MDSCs or CD11b+Ly6G+Ly6Clowmyeloid cell inflammation in autoimmune diseases and how to reform its suppressive function are also worth further exploration.
The incidence rate of the CIA model is about 90%, the clinical score of arthritic mice usually exceeds 6, and the proinflammatory characteristics of PMN-MDSCs may be covered when the cells are directly transferred to the established CIA mice. We tried to keep the recipient mice in a lower inflammatory state to reveal the inflammatory role of transferred PMN-MDSCs. Therefore, we only administered the recipient mice the first immunization; the PMN-MDSCs from established CIA mice (score > 6) were adoptively transferred to the recipient mice on days 21 and 28, and we found that the onset of the disease was accelerated and the swelling of the joints was also exacerbated. Thus, the inflammatory role of PMN-MDSCs is important in CIA pathogenesis.
This study had some limitations. BAFF is crucial for B cell survival and development(20-23), however, in our study, we only focused on BAFF expression in PMN-MDSCs, while other immune cells or FLS expression were not detected. Whether PMN-MDSCs were the main source of BAFF in the joints and spleens is not yet known. Apart from TNF-๐›‚, B cell produced a variety of cytokines, such as IL-10, IL-6, GM-CSF and so on, which play important role in the pathogenesis of RA(34, 35), and B cell also had subtypes, such as plasma B cell(36), marginal zone B cell(37), germinal center B cell(38) and so on, however, in this work we did not explore the regulation of PMN-MDSC to B cell differentiation and other cytokines production. Further studies were needed to understand how PMN-MDSC regulates B cells.
In our study, we found that PMN-MDSCs increased B cellsโ€™ TNF-๐›‚ secretion via BAFF/BTK/NF-๐›‹B signaling pathway, which indicated the importance of BAFF and BTK in the pathogenesis of CIA and provided additional evidence for the application anti-BAFF monoclonal antibody and BTK inhibitor in the treatment of RA.